1. Field of the Invention
This invention generally relates to ways and means for the disposal by combustion of grease, scum, and other organic floatage such as are produced during waste water treatment processes; more particularly, this invention relates to ways and means for conditioning and burning mixtures of such materials which have a fairly high water content.
2. State of the Art
Municipal and industrial waste water treatment processes usually produce not only sedimentary wastes, such as sedementary sewage sludge, but also floating wastes which usually comprise, for example, grease, oil, scum and other materials which float to the water surface. Such floatage is particularly found in primary treatment tanks. Because such materials are capable of being removed from the water surface by a skimming process, they are hereinafter referred to as "skimmings." A blade, chain-driven scoop or other devices can be used to skim the water surface; sometimes a weir is used and the water and/or weir height controlled so the skimmings run off over the weir.
The skimmings comprise a large percentage of water (usually over 50%), together with the aforementioned grease, scum, wood chips and other floatage. Usually the skimmings comprise organic materials, but sometimes plastic pieces and synthetic filaments are also included. Such very non-homogeneous skimmings present difficult handling and disposal problems.
Known methods of disposal of the skimmings include burial, chemical digestion and burning. Equipment for burning the skimmings generally comprises a combustor or burner designed to handle a liquid waste through a steam, air, or mechanical atomizing nozzle. The liquid feed is atomized as finely as possible to present the greatest surface area for mixing with combustion air. Sometimes a secondary incineration chamber is provided and that may comprise, for example, a vertically-arranged cylinder which acts as its own stack.
It is also known to feed skimmings directly to sludge incinerators such as multiple-hearth furnaces, sometimes obviating the necessity of using auxiliary fuels to burn the solid sludge and at the same time disposing of the skimmings. However, due to their high thermal values, the skimmings must be fed slowly to the incinerator and then must be burned immediately or else there is the possibility of a subsequent uncontrollable flare-up. Such flare-ups can exceed the temperature capacity of the incinerator and, consequently, can destroy the furnace exhaust ducting and the like or even create an explosion.
Because the skimmings are not an evenly-flowing medium and often include large solid pieces or fibrous and synthetic strands, it is difficult to feed the skimmings to an incinerator while achieving therein an even distribution. To avoid clogging the supply system, the skimmings generally are introduced to the incinerator through a single large pipe. However, the flow from the single large pipe does not evenly distribute the skimming and, consequently, there is uneven burning inside the incinerator. As a result, ash stalactites sometimes form in the furnace and eventually interfere with its operation. Furthermore, grease may soak into the refractory brick of the furnace so that, after a period of time, the bricks are destroyed.